The present invention relates generally to a process for the production of hollow bodies from thermoplastic material and more particular such hollow bodies whose wall at least partially comprises foamed thermoplastic material of a multi-layer structure, by an extrusion blowing procedure.
In one form of process for producing hollow bodies with a multi-layer or laminate wall structure, as disclosed for example in U.S. patent specification No. 3 901 958, a preform comprising foamable material is extruded into a cavity which is under an increased pressure above atmospheric pressure, such that foaming of the layer of foamable material of the preform is substantially hindered. A blow molding mold is also disposed within the above-mentioned cavity, and the preform is expanded within the blowing mold in a non-foamed condition. Thereupon the pressure in the cavity accommodating the hollow molded article and in the hollow body itself is reduced to a level which permits foaming of the layer consisting of foamable material.
One of the aspects on which that process is based is that it is not possible, or is possible only at the expense of major difficulties, to expand a preform which has already been foamed up. Those difficulties are essentially to be attributed to the point that the foamed material of a preform which is still in a thermoplastic condition has regions which exhibit different strength and expandability values, over the length and periphery of the preform. The result of that is that weak points are formed under the effect of the internal pressure within the preform, in the wall of the preform or the hollow molded article which is to be produced therefrom. In many circumstances such weak points cause the wall of the preform or the molded article produced therefrom to tear open, giving rise to wastage. The above-indicated previous process takes account of that fact, but at the expense of incurring serious disadvantages in terms of operating procedure and the equipment involved. Thus, maintaining an increased pressure in the above-mentioned cavity in order to prevent the foamable material from foaming and expanding in the first phase of the production operation complicates the operating procedure involved. A similar point arises in regard to restoring the increased pressure in the cavity, before beginning the next working cycle in the process. In addition, a high level of apparatus expenditure is required in that connection, in particular for the purposes of delimiting relative to the atmosphere the above-mentioned cavity which accommodates the blowing mold and the associated components thereof and which makes it possible to build up the increased pressure required. Furthermore, the fact that the wall of the preform comes into contact with the wall of the mold prior to the beginning of the foaming process necessarily results in the material suffering from a cooling effect, which can have an adverse effect on the foaming process, that is to say, the foam cannot be formed unimpededly and with total freedom to expand. Finally, the above-described process does not take account of the fact that hollow bodies which are produced by an extrusion blowing procedure are provided with welded seams, the production of which may require special precautionary steps to be taken, by virtue of the presence of layers of foamed material in the hollow body.
The above-described process also provides for producing and expanding a two-layer preform in which the outer layer comprises non-foamable thermoplastic material. However that layer serves exclusively for decorative purposes and makes no contribution to structural integrity of the molded article.